Multi-layer opaque film structures tailored to end-use requirements

ABSTRACT

An opaque, biaxially oriented polymeric film structure, produced by orienting the film to a degree which provides low light transmission and improved machinability. The film structure includes a thermoplastic polymer matrix core layer having a first surface and a second surface, within which is located a strata of voids; positioned at least substantially within a substantial number of the voids is at least one spherical void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by the particle being substantially less than the volume of the void; the population of the voids in the core being such as to cause a significant degree of opacity, the core layer including a light absorbing pigment of lamellar morphology; a first non-voided thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the first intermediate layer adhering to the first surface of the core layer; and a second non-voided thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the second intermediate layer adhering to the second surface of the core layer; wherein the level of light transmission of the film is less than 10% and the degree of orientation is about 8 TDO by less than about 5 MDO.

FIELD OF THE INVENTION

This invention relates to the field of polymer films of enhanced opacityand to a method of making such films. More particularly, the inventionrelates to a biaxially oriented composite film structure having improvedproperties.

BACKGROUND OF THE INVENTION

In the packaging of certain types of foods, such as snack foods likepotato chips, cookies and the like, it is common practice to employ amulti-layer film. A desirable property in such a packaging film is anopacity which protects the packaging product from deterioration causedby exposure to light. In particular. It has been found that certainwavelengths of light, up to about 450 nm cause increased spoilage insuch packaged products. Even when a degree of opacity is present in thefilm, spoilage occurs if the film allows passage of some light.

It is known in the art that thermoplastic polymers can be loaded withinert fillers, cast into films, and thereafter stretched to formoriented thermoplastic films. While this statement is generally true, itmust be realized that the particular components employed and thespecific process parameters employed, particularly when control isdifficult, can result in significantly different end products orcontribute to the success or failure of obtaining a desired result. Asan example, U.S. Pat. No. 4,118,438, discloses the use of some materialssimilar to those contemplated by the present invention, however, theobject of U.S. Pat. No. 4,118,438 is diametrically opposed to the objectof the present invention. U.S. Pat. No. 4,118,438 is concerned with atransparent polypropylene film containing fine particles of anincompatible polymer dispersed therein. The film disclosed exhibitssurface projections caused by the dispersed particles and the patenteesmaintain that this gives the transparent film non-blockingcharacteristics. In U.S. Pat. Nos. 3,697,367 and 3,795,720, there isdisclosed a process for preparing an uniaxially oriented mixed polymersystem. The resulting material has utility as a paper substitute and canbe formed into fibers for making sheet paper.

Oriented opaque film compositions are known in the art. U.S. Pat. No.4,377,616 discloses an opaque biaxially oriented polymer film structurecomprising a thermoplastic polymer matrix core possessing numerousvoids, a substantial number of which contain at least one sphericalvoid-initiating particle, and transparent thermoplastic skin layersadhering to the surfaces of the core layer. The unique structure of thecore layer imparts a much higher degree of opacity, possibly due to theeffects of light scattering, than that possible from the use of anopacifying pigment alone. In U.S. Pat. No. 4,377,616, the film isprepared by melting a mixture of a major proportion of a film formingpolymer such as polypropylene and a minor proportion of an incompatiblepolymer which has a higher melting point, at a temperature sufficient tomelt the incompatible polymer and to dispense it in the film formingpolymer, extruding the mixture into a film and biaxially orienting thefilm. The dispersed incompatible polymer provides sites for theformation of voids surrounding the polymer particles. These voidsprovide opacity and give the film an attractive pearlescent sheen.

U.S. Pat. No. 4,632,869 discloses an opaque, biaxially oriented filmstructure having a polymer matrix with a strata of voids, the voidscontaining spherical void-initiating particles of polybutyleneterephthalate. The structure may also include thermoplastic skin layersand the film can include from about 1% to 3% b weight of a pigment suchas TiO₂ or colored oxides.

U.S. Pat. No. 4,758,462 also discloses an opaque, biaxially orientedfilm with a cavitated core and transparent skin layers. Colored lightabsorbing pigments such as carbon black or iron oxide are added to thecore and/or the skins in an amount of about 2 to 12 weight % to decreaselight transmission through the film.

U.S. Pat. No. 4,652,489 discloses an oriented, sealable, opaquepolyolefin multi-layer film with a core layer containing vacuoles, asealable surface layer, and a non-sealable surface layer whichincorporates a slip agent such as a polydiorganosiloxane.

U.S. Pat. No. 4,741,950 discloses a differential opaque polymer filmwith a core layer containing numerous microscopic voids, arough-appearing wettable first skin layer which contains an antiblockingagent such as silica, silicate, clay, diatomaceous earth, talc andglass, and a second wettable skin layer with a smooth appearance whichmay be metallized. TiO₂ may be present in the core and/or first skinlayer. The film allows a light transmission of 24%.

U.S. application Ser. No. 07/324,134, a co-inventor of which is also theinventor of the present invention, discloses a non-symmetricallylayered, highly opaque, biaxially oriented polymer film with a corecontaining numerous microscopic voids and at least about 1% by weight ofopacifying compounds; a first skin layer on one surface of the corecontaining up to about 12% by weight of inorganic particulate material;and a second skin layer on the other surface of the core. U.S.application Ser. No. 07,324,134, also discloses the benefit whichaccrues from the addition of inorganic particles such as titaniumdioxide to whiten the surface of the outer skin layer of the filmstructure. The increase in whiteness yields an excellent surface forprinted graphics. A further benefit resulting from increased whitenessin the outer skin layer of the film is that it permits the printing oflaminated or unlaminated film structures without the need for while ink,offering a significant savings to the end user.

While films which employ titanium dioxide-whitened outer skin layersprovide the aforementioned benefits, such films can also yield certainundesirable characteristics. These characteristics stem from the factthat titanium dioxide (TiO₂) is quite abrasive and, when present on thesurface of a film, may result in excessive wear of expensive printingand coating gravure roll surfaces, as well as any other surface which iscontacted by such a film. Another problem which arises from the use ofTiO₂ in the outer skin layers of such films is that fine deposits arelaid on converting machinery, extruder die lips, treater bar exhausts,etc. Also, appearance problems caused by streaks on the film slippage onstretching either by roll or tentering can result.

U.S. application Ser. No. 07/699,864, now U.S. Pat. No. 5,091,236, theinventors of which are also the inventors of the present invention,discloses multi-layer opaque, biaxially oriented polymeric filmstructures which avoid the problems associated with films employingtitanium dioxide-whitened outer skin layers. The film structuresdisclosed include (a) a thermoplastic polymer matrix core layer having afirst surface and a second surface, within which is located a strata ofvoids; positioned at least sbustantially within a substantial number ofthe voids is at least one spherical void-initiating particle which isphase distinct and incompatible with the matrix material, the void spaceoccupied by the particle being substantially less than the volume of thevoid, with one generally cross-sectional dimension of the particle atleast approximating a corresponding cross-sectional dimension of thevoid; the population of the voids in the core being such as to cause asignificant degree of opacity; (b) at least one thermoplastic polymerintermediate layer having a first surface and a second surface, thesecond surface of the intermediate layer adhering to at least the firstsurface of the core layer, the intermediate layer including up to about12% by weight of titanium dioxide contact pigment; and (c) a titaniumdioxide-free, non-voided thermoplastic skin layer adhering to the firstsurface of the intermediate layer, the void-free skin layer and theintermediate layer together being of a thickness such that the outersurface of the skin core layer does not, at least substantially,manifest the surface irregularities of the matrix core layer. U.S.application Ser. No. 07/699,864 is hereby incorporated by reference inits entirety for all that it discloses.

Despite these significant advances in the art, many of the filmsdescribed within the above-referenced patent literature prove to besomewhat fragile in certain end-use applications. While highly cavitatedor voided films are ideally suited for certain applications, such asoverwrapping biscuits and the like, in other applications such as thoseemploying Vertical Form Fill and Seal (VFFS) packaging machinery, aswell as some which employ Horizontal Form Fill and Seal (HFFS)machinery, the forming collars of the machine often damage or shear thepackage being formed and filled. In addition, films converted intopackages which are filled with both product and air, so as to provide anair cushion for product protection, often benefit from the use of a lessfragile film. However certain means employed to provide a less fragilevoided film can reduce the film's pleasing aesthetic appearance or maychange the ability of the film to inhibit the transmission of lighttherethrough.

Therefore, what is needed is a less fragile, opaque film structure whichprovides an improved range of machining operability, while maintainingits appearance characteristics, strength and stiffness and a process forthe production of such a film structure.

SUMMARY OF THE INVENTION

The film structure of the present invention is an opaque, biaxiallyoriented polymeric film structure produced by orienting the film to adegree which provides low light transmission and improved machinability.The film structure includes (a) a thermoplastic polymer matrix corelayer having a first surface and a second surface, within which islocated a strata of voids; positioned at least substantially within asubstantial number of the voids is at least one sphericalvoid-initiating particle which is phase distinct and incompatible withthe matrix material, the void space occupied by the particle beingsubstantially less than the volume of the void, the population of thevoids in the core being such as to cause a significant degree ofopacity, the core layer including alight absorbing pigment of lamellarmorphology, (b) a first non-voided thermoplastic polymer intermediatelayer having a first surface and a second surface, the second surface ofthe first intermediate layer adhering to the first surface of the corelayer, and (c) a second non-voided thermoplastic polymer intermediatelayer having a first surface and a second surface, the second surface ofthe second intermediate layer adhering to the second surface of the corelayer, wherein the level of light transmission of the film is less than10% and the degree of orientation is about 8 TDO by less than about 5MDO.

More preferred is a five-layer film structure, incorporating theabove-described (a), (b) and (c) layers, and further including (d) afirst non-voided thermoplastic skin layer adhering to said first surfaceof said first intermediate layer; and (e) a second non-voidedthermoplastic skin layer adhering to said first surface of said secondintermediate layer, the void-free skin layers being of a thickness suchthat the outer surface of the skin layers do not, at leastsubstantially, manifest the surface irregularities of the matrix corelayer.

The skin layers (d) and/or (e) can be simple, economical thinencapsulating layers or they can be more elaborate heat sealable layers.

Also provided is a process for preparing an opaque, biaxially orientedpolymeric film structure of low light transmission and improvedmachinability, comprising the steps of: (a) mixing a major proportion ofa first thermoplastic polymeric material with a minor proportion of afirst material of higher melting point or having a higher glasstransition temperature than the first thermoplastic polymeric materialto produce a core layer mixture and a minor amount of a light absorbingpigment of lamellar morphology; (b) heating the core layer mixtureproduced in step (a) to a temperature of at least above the meltingpoint of the first thermoplastic polymeric material; (c) dispersing thefirst material of higher melting point or higher glass transitiontemperature of the mixture produced in step (a) uniformly throughout themolten first thermoplastic polymeric material in the form ofmicrospheres; (d) mixing a second thermoplastic polymeric material toproduce a first intermediate layer mixture; (e) heating the intermediatelayer mixture produced in step (d) to a temperature of about the meltingpoint of the second thermoplastic polymeric material; (f) mixing a thirdthermoplastic polymeric material to produce a second intermediate layermixture; (g) heating the second intermediate layer mixture produced instep (f) to a temperature of about the melting point of the secondthermoplastic polymeric material; and (h) forming a biaxially orientedcoextruded film structure from the core layer mixture, the firstintermediate layer mixture and the second intermediate layer mixture,the forming step conducted at a temperature and to a degree to form astrata of opacifying voids within the core layer; wherein the degree oforientation is about 8 TDO by less than about 5 MDO.

Accordingly, it is an object of the present invention to provide a filmstructure of high opacity.

It is another object of the present invention to provide a film withimproved processing characteristics.

It is a further object of the present invention to provide a filmstructure having an improved range of machinability.

It is yet another object of the present invention to provide a filmwhich may be bonded to a wide variety of substrates and coating.

It is a yet further object of the present invention to provide amulti-layer film structure of pleasing appearance which does notcontribute to operability problems caused by highly fragile films.

Other objects and the several advantages of the present invention willbecome apparent to those skilled in the art upon a reading of thespecification and the claims appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a method for determining percent lighttransmission.

FIG. 2 is a schematic diagram of a method for determining percentopacity.

DETAILED DESCRIPTION OF THE INVENTION

In order to achieve the unique film structure of the present invention,it is important that a particular thickness relationship exist betweenthe thickness dimension of the core and the thickness of the skinlayers. It is preferred that the core thickness be from about 60 toabout 95% of the overall structure with about 65-90% preferred. This incombination with the population and configuration of the voids in atotal structure of at least about 1.0 mil thick, will materiallycontribute to the overall degree of opacity of the structure. Likewise,by maintaining the thickness of the skin layers within particular rangesin relation to the overall structure and to the thickness of the corelayer, the overall combination results in unique advantages.Intermediate layer (b), adhering to the first surface of core layer (a)and intermediate layer (c) adhering to the second surface of core layer(a) each have a thickness of from about 5 to about 30% of the overallstructure, with a thickness of about 5 to about 15% preferred.Intermediate layers (b) and (c) are formed from a thermoplastic polymerand either or both may include a migratory slip agent and a migratoryantistatic agent, when encapsulating skin layers (d) and (e) areemployed, as will be described in more detail hereinbelow. Once such afive layer structure is formed, the slip and antistatic agents migrateto the outer surface of skin layers (d) and/or (e) to reduce the surfacecoefficients of friction thereof. The intermediate layer serves animportant function in reducing water vapor transmission rate (WVTR) andmay also contain TiO₂ as a contact whitening agent. Skin layers (d) and(e), which preferably are TiO₂ -free, adhering to the surfaces of theintermediate layers not in contact with the core layer, have thicknessesof from about 0.10% to about 5.0% of the overall structure withthicknesses of from about 0.20% to about 3.0% preferred. The relativethinness of this layer adds to economy in production especially when thelayer is an expensive heat-sealable material. A preferred five-layerstructure might include, for example, a core layer with a thickness ofabout 75% of the overall structure with intermediate layer (b) and (d)having thicknesses of about 10% each and skin layers (c) and (e) havingthicknesses of about 2.5% each.

The core is a thermoplastic polymer matrix material within which islocated strata of voids. From this it is to be understood that the voidscreate the matrix configuration.

The films of the present invention have high opacity and low lighttransmission. A distinction should be made between opacity and lighttransmission. Opacity is the opposite of transparency and is a functionof the scattering and reflection of light transmitted through the film.Opacity is the ability, for example, to block out writing below it.Light transmission is a function of light passing more directly throughthe film.

Referring now to FIG. 1, the percent light transmission through a filmis determined by using light source 2 to transmit light rays 3 directlythrough film 4 and measuring at light sensor 5, value T₂ which is theamount of light which is transmitted through film 4. The amount of lightrays 3 which can be directly transmitted, value T₁, is determined bymeasuring the light 3 directly transmitted by light source 2 with nointervening film. The percent light transmission through the film canthen be determined using the formula: ##EQU1## where: T₂ =lighttransmitted through a film; and T₁ =light directly transmitted.

Referring now to FIG. 2, for a measure of percent opacity of a film,light source 2 transmits light through film 4 onto a white surface 9 andthe same procedure used to project light onto a black surface 10. Withboth white and black surfaces, measurement at light sensor 5 is of allof the following: light reflected off the upper surface of the film 6;light transmitted through the film and reflected by the white or blacksurfaces 7 on the side of the film opposite from the light source; and,light scattered by the film 8.

The percent opacity of the film can then be determined using theformula: ##EQU2## where: R_(w) =Reflected light+scattered light+lighttransmitted through the film and reflected off a white surface; andR_(B) =Reflected light+scattered light+light transmitted through thefilm and reflected off a black surface.

Accordingly, a highly reflective film may provide high opacity whileallowing light transmission. This is because percent light transmissionis not the equivalent to percent opacity. Light transmission is theamount of light passing directly through the film. To prevent foodspoilage decreased light transmission is desirable.

In forming the core layer, as in U.S. Pat. No. 4,377,616, the disclosureof which is incorporated herein by reference in its entirety, a masterbatch technique can be employed by either in the case of forming thevoid initiating particles in situ or in adding preformed spheres to amolten thermoplastic matrix material. After the formation of a masterbatch, appropriate dilution of the system can be made by addingadditional thermoplastic matrix material until the desired proportionsare obtained. However, the components may also be directly mixed andextruded instead of utilizing a master batch method.

The void-initiating particles which are added as filler to the polymermatrix material of the core layer can be any suitable organic orinorganic material which is incompatible with the core material at thetemperature of biaxial orientation such as polybutylene terephthalate,nylon, solid or hollow preformed glass spheres, metal beads or spheres,ceramic spheres, calcium carbonate, etc.

The polyolefin contemplated as the core material includes polypropylene,polyethylene, polybutene and copolymers and blends thereof. Particularlypreferred is an isotactic polypropylene containing at least about 80% byweight of isotactic polypropylene. It is also preferred that thepolypropylene have a melt flow index of from about 2 to 10 g/10 min.

It is preferred that the average diameter of the void-initiatingparticles be from about 0.1 to about 10 microns. These particles may beof any desired shape although it is preferred that they be substantiallyspherical in shape. This does not mean that every void is the same size.IT means that, generally speaking, each void tends to be of like shapewhen like particles are used even though they vary in dimensions. Thesevoids may assume a shape defined by two opposed and edge contactingconcave disks.

Experience has shown that optimum characteristics of opacity andappearance are obtained when the two average major void dimensions aregreater than about 30 microns.

The void-initiating particle material, as indicated above, should beincompatible with the core material, at least at the temperature ofbiaxial orientation.

The core has been described above as being a thermoplastic polymermatrix material within which is located a strata of voids. From this itis to be understood that the voids create the matrix configuration. Theterm "strata" is intended to convey the understanding that there aremany voids creating the matrix and the voids themselves are oriented sothat the two major dimensions are aligned in correspondence with thedirection of orientation of the polymeric film structure. After eachvoid has been formed through the initiation of the described particle,the particle generally contributes little else to the system. This isbecause its refractive index can be close enough to the matrix materialthat it makes no contribution to opacity. When this is the case, theopacity is principally a function of the light scattering effect whichoccurs because of the existence of the voids in the system.

A typical void of the core is defined as having major dimensions X and Yand minor dimension Z, where dimension X is aligned with machinedirection orientation, dimension Y is aligned with transverse directionorientation and dimension Z approximately corresponds to thecross-sectional dimension of the spherical particle which initiated thevoid.

It is a necessary part of the present invention that orientationconditions be such that the X and Y dimensions of the voids of the corebe major dimensions in comparison to the Z dimension. Thus, while the Zdimension generally approximates the cross-sectional dimension of thespherical particle initiating the void, X and Y dimensions must besignificantly greater.

By way of illustration, room temperature biaxial orientation of apolypropylene matrix containing polybutylene terephthalate (PBT) spheresof the size and amount contemplated herein, could not produce theclaimed structure. Either void splitting will occur, or, voids ofinsignificant size would result. Polypropylene must be oriented at atemperature significantly higher than its glass transition temperature.The temperature conditions must permit X and Y to be at least severalmultiples of the Z dimension without void splitting at least to anysignificant degree. If this is accomplished, optimum physicalcharacteristics, including low water vapor transmission rates and a highdegree of light scattering are obtained without void splitting or filmfibrillating.

As indicated above, the matrix polymer and the void initiating particlemust be incompatible and this term is used in the sense that thematerials are two distinct phases. The spherical void initiatingparticles constitute a dispersed phase through the lower melting polymerwhich polymer will, ultimately, upon orientation, become a void-filledmatrix with the spherical particles positioned somewhere in the voids.

As a result of the biaxial orientation of the film structure herein, inaddition to opacifying the core layer of the structure, the orientationimproves other physical properties of the composite layers such asflex-crack resistance, Elmendorff tear strength, elongation, tensilestrength, impact strength and cold strength properties. The resultingfilm can have, in addition to a rich high quality appearance andexcellent opacifying characteristics, low water vapor transmission ratecharacteristics and low oxygen transmission rate characteristics. Thismakes the film ideally suited for packaging food products includingliquids. The film also has attractive utility as a decorative wrapmaterial.

It is believed that because of comparative sphericity of thevoid-initiating particles, the voids are closed cells. This means thatthere is virtually no path open from one side of the core the otherthroughout which liquid or gas can transverse.

The opacity and low light transmission of the film may be furtherenhanced by the addition to the core layer of from about 1% by weightand up to about 10% by weight of opacifying compounds, which are addedto the melt mixture of the core layer before extrusion. Opacifyingcompounds which may be used include iron oxides, carbon black, aluminum,TiO₂, and talc. The opacifying compounds do not contribute to voidformation.

Preferred for use in the core layer are pigment particles of a lamellarmorphology. The pigment particles of lamellar morphology (lamellarpigment) may comprise an organic pigment, preferably graphite, or aninorganic pigment, such as a silicate, preferably a silicate which has apronounced tendency to cleave in one preferred planar direction, forexample, a mica. Graphite is a particularly preferred lamellar pigment.The lamellar pigment should have an average particle size which, ofitself, is insufficient to cause voiding of the matrix; this will dependon the nature of the matrix and the processing conditions, especiallydeformation temperature and the amount of the pigment in the matrix. Theterm "voiding of the matrix" as used herein designates creating a spacewithin the matrix. However, good results are obtained when the lamellarpigment has an average particle size from 0.2 to 2.0 micrometers,preferably from 0.5 to 1.0 micrometers. The lamellar pigment may bepresent in an amount from 0.2 to 12 wt. %, suitably from 0.5 to 5.0 wt %of the film preferably from 1.0 to 2.0 wt. % of the film.

The polyolefin contemplated as the material for use in formingintermediate layers (b) and (c) includes polypropylene, polyethylene,polybutene and copolymers and blends thereof. As was the case for thecore layer, particularly preferred is an isotactic polypropylenecontaining at least about 80% by weight of isotactic polypropylene. Itis also preferred that the polypropylene have a melt flow index of fromabout 2 to 10 g/10 m.

To reduced the coefficient of friction at the surface of the skin layeror layers, the use of certain slip agents in conjunction with certainantistatic agents in the intermediate layer, when each such agentpossesses migratory properties, can be employed. Sufficient aging timemust be provided to permit the combination of agents to migrate to theouter surfaces of the film. By "migratory" is meant the property ortrait of bleeding through the polymeric matrices of the intermediate andskin layers so at least an effective amount of the agents reside on thesurface of the skin layer to reduce the surface coefficient of frictionthereof.

While a variety of migratory antistatic agents are availablecommercially, preferred for use in the practice of the present inventionis the group of ethoxylated amines and ethoxylated amides. Ethoxylatedamines are available from the Humco Chemical Division of Whitco ChemicalCorp. under the trademark of Kemamine®, from the Noury Chemical Companyunder the trademark of Armostat® and from other sources. Ethoxylatedamides are available from Akzo Chemie America under the trademark ofEthmid®, from the Oxynol Chemical Company under the tradename of Oxynol®and from other sources. Particularly preferred for their migratoryproperties is the group of ethoxylated amines.

Likewise, a wide variety of migratory slip agents are availablecommercially, some of which are disclosed in U.S. Pat. No. 4,510,281,the contents of which are hereby incorporated by reference for thosedetails. Preferred for use in the practice of the present invention isthe group of commercially available amine- and amide-based slip agents,with erucamide being most particularly preferred.

The range of antistatic agent levels useful in the practice of thepresent invention is from about 500 ppm to about 2000 ppm of theintermediate layer mixture, with about 1000 ppm to about 1500 ppm beingparticularly preferred. It is important to note that when themulti-layer films of the present invention are to come in contact withfoods, governmental regulatory agencies generally limit the amount ofadditives materials which can be employed therein. For example, amine-and amide-based antistatic materials are limited to approximately 1200ppm of the intermediate layer mixture. Nevertheless, this limitationdoes not inhibit the ability of the agent to cooperate with themigratory slip agent to yield the beneficial properties ascribedthereto. The range of slip agent levels useful in the practice of thepresent invention is from about 200 ppm to about 2000 ppm of theintermediate layer mixture, with about 800 ppm to about 1500 ppm beingparticularly preferred. For example, when seeking to form a 0.8 milthickness multi-layer structure, in accordance with the presentinvention, about 1500 ppm of erucamide is required. When forming a 1.6mil thickness structure, about 800 ppm of erucamide is required. Amine-and amide-based slip agents, such as the most particularly preferredagent, erucamide, are permitted by governmental regulatory agencies tobe present at significantly higher levels than necessary to perform thefunction intended by the present invention.

It has been discovered that the combined use of a migratory antistaticagent with a migratory slip agent results in a synergistic reduction inthe coefficient of friction. It is thought that this occurs due to thefact that a film having poor slip characteristics will tend to generateand store more static charges and a film having a high level of storedstatic charge will tend to cling more to any surface with which it comesin contact with, thus exhibiting a higher coefficient of friction.

Moreover, it has been discovered that the placement of the migratoryslip agent and the migratory antistatic agent in the intermediate layers(b) and (c) produces a much higher level of effectiveness than can beachieved through their placement within the core layer (a) or the skinlayers (d) and (e). With regard to the placement of the agents in thecore layer, this is due to the fact that the migratory slip agent andthe migratory antistatic agent will tend to cling to the interiorsurfaces of the voids formed within the core layer. With regard to theplacement of the agents in the skin layer or layers, it is the volatilenature of the agents themselves which tend to limit their effectiveness.As described herein, the temperatures required to form the multi-layerstructures of the present invention will tend to "boil" away the agents,inhibiting their ability to function as intended.

The opacity, whiteness and low light transmission of the film may befurther enhanced by the addition to intermediate layers (b) and (c) ofTiO₂ in amount of from about 1% by weight and up to about 10% by weight,which is added to the melt mixture of the intermediate layer beforeextrusion. Preferably, the intermediate layers contain from about 2% byweight to 6% by weight of TiO₂. Additionally, the intermediate layersmay also contain talc. The whiteness resulting from the inclusion ofTiO₂ provides an excellent surface for graphics. Furthermore, thewhiteness allows printing of laminated or unlaminated structures withoutrequiring white ink.

Layers (d) and (e) are thin skin layers applied to the surfaces ofintermediate layers (b) and (c) which are not in contact with the corelayer (a). Layers (d) and (e) are preferably of a material having a lowWVTR. This layer may consist of a propylene; high density polyethylene;linear low density polyethylene; block copolymer of ethylene andpropylene; random copolymer of ethylene and propylene; other ethylenehomopolymer, copolymer, terpolymer; or blends thereof. The homopolymercontemplated herein is formed by polymerizing the respective monomer.This can be accomplished by bulk or solution polymerization, as thoseskilled in the art would plainly understand. One of the preferredmaterials for layers (d) and (e) is isotactic polypropylene. Skin layers(d) and (e) are of a thickness sufficient to encapsulate theintermediate layers, and, when TiO₂ is employed in intermediate layers(b) and (c), the desired effect of reduced processing machinery wearproblems associated with TiO₂ -containing outer layers is achieved.Moreover, the combination of intermediate layer (b) and skin layer (d)and intermediate layer (c) and skin layer (e) provide a thickness suchthat the outer surface of each skin layer does not, at leastsubstantially, manifest the surface irregularities of the matrix corelayer (a).

The copolymer contemplated herein for skin layers (d) and/or (e) can beselected from those copolymers typically employed in the manufacture ofmulti-layered films. For example, a block copolymer of ethylene andpropylene is formed by sequential polymerization of the respectivemonomers. The feeding of the monomers in forming a block copolymer iscontrolled so that the monomer employed in one stage of the sequentialpolymerization is not added until the monomer employed in the precedingstage has been at least substantially consumed thereby insuring that theconcentration of the monomer remaining from the preceding stage issufficiently low to prevent formation of an excessive proportion ofrandom copolymer. Also, as indicated above, a random copolymer ofethylene and propylene can be advantageously employed to form skinlayers (d) and/or (e).

The contemplated terpolymers which may be used for skin layers (d)and/or (e) are comparatively low stereoregular polymers. The terpolymerscan have a melt flow rate at 446° F. ranging from about 2 to about 10grams per 10 minutes and preferably from about 4 to about 6 grams per 10minutes. The crystalline melting point can range from about less than250° F. to somewhat greater than 371° F. The terpolymers willpredominate in propylene, and the ethylene and 1-butene monomers can bepresent in approximately from 0.3:1-1:1 mole percentage in relation toeach other.

If desired, the exposed surface of skin layers (d) and/or (e) can betreated in a known and conventional manner, e.g., by corona discharge toimprove its receptivity to printing inks and/or its suitability for suchsubsequent manufacturing operations as lamination.

The exposed treated or untreated surface of layers (d) and/or (e) mayhave applied to it, coating compositions or substrates such as anotherpolymer film or laminate; a metal foil such as aluminum foil; cellulosicwebs, e.g. numerous varieties of paper such as corrugated paperboard,craft paper, glassine, cartonboard; nonwoven tissue, e.g., spunboundedpolyolefin fiber, melt-blown microfibers, etc. The application mayemploy a suitable adhesive, e.g., a hot melt adhesive such as lowdensity polyethylene, ethylene-methacrylate copolymer, water-basedadhesive such as polyvinylidene chloride latex, and the like.

Layers (d) and/or (e) may also include up to about 1% by weight, withabout 500 ppm to about 5000 ppm preferred and 1000 ppm most preferred,of inorganic particles, such as amorphous silica or talc to provideantiblock properties.

Skin layers (d) and/or (e) can also be fabricated from any of the heatsealable copolymers, blends of homopolymers and blends of copolymer(s)and homopolymer(s) heretofore employed for this purpose. Illustrative ofheat sealable copolymers which can be used in the present invention areethylene-propylene copolymers containing from about 1.5 to about 10, andpreferably from about 3 to about 5 weight percent ethylene andethylene-propylene-butene terpolymers containing from about 1 to about10, and preferably from about 2 to about 6 weight percent ethylene andfrom about 30 to about 97, and preferably from about 88 to about 95weight percent propylene. Heat sealable blends of homopolymer which canbe utilized in providing layers (c) and/or (e) include from about 1 toabout 99 weight percent polypropylene homopolymer, e.g., one which isthe same as, or different from, the polypropylene homopolymerconstituting core layer (a) blended with from about 99 to about 1 weightpercent of a linear low density polyethylene (LDPE). If layers (d)and/or (e) are heat-sealable, corona or flame treatment of layers (d)and/or (e) is not required.

Heat sealable blends of copolymer(s) and homopolymer(s) suitable forproviding layers (d) and/or (e) include: a blend of from about 5 toabout 19 weight percent of polybutylene and from about 95 to about 81weight percent of a copolymer of propylene (80 to about 95 mole percent)and butylene (20 to about 5 mole percent); a blend of from about 10 toabout 90 weight percent of polybutylene and from about 90 to about 10weight percent of a copolymer of ethylene (2 to about 49 mole percent)and a higher olefin having 4 or more carbon atoms (98 to about 51 molepercent); a blend of from about 10 to about 90 weight percentpolybutylene and from about 90 to about 10 weight percent of a copolymerof ethylene (10 to about 97 mole percent) and propylene (90 to about 3mole percent); and, a blend of from about 90 to about 10 weight percentof polybutylene, and from about 10 to about 90 weight percent of acopolymer of propylene (2 to about 79 mole percent) and butylene (98 toabout 21 mole percent).

If skin layers (d) and/or (e) are not heat sealable, and that propertyis desired on one or both of those surfaces, then a heat sealable layer(f) may be applied to one or both of those surfaces. Heat sealable layer(f) may be, for example, vinylidene chloride polymer or an acrylicpolymer; or it may be coextruded from any of the heat sealable materialsdescribed herein. Vinylidene chloride polymer or acrylic polymercoatings are preferred materials which may be applied to the exposedexterior surfaces of the skin layers.

It is preferred that all layers of the multi-layer film structures ofthe present invention be coextruded. Thereafter, the film is biaxiallyoriented. For example, when employing polypropylene for the core matrixand the skin layers and employing PBT as the void initiating particles,a machine direction orientation may be from about 3 to about 8 and atransverse orientation may be from 4 to about 10 times at a drawingtemperature of about 100° C. to 170° C. to yield a biaxially orientedfilm. A preferred film thickness is from about 0.5 mil to about 3.5mils.

Whiteness and opacity is created in the structures of the presentinvention by biaxially orienting first in the machine direction (MDO)and subsequently in the transverse direction (TDO). As indicated, in theorientation process the stretching force causes the thermoplasticpolymer to part around the finely dispersed void-initiating particlesand from oval shaped cavities or microvoids, supported by the particles.The whiteness and opacity of the resulting cavitated film depend on thefollowing factors:

Polymer volume, i.e., polygage

Core and skin ratios

Core composition

The degree of cavitation as indicated by the film density g/cc ascalculated by dividing the weight of one square meter by the opticalgage (μ)

The higher the cavitation, that is the lower the film density, a whiterand more opaque film results; the lower the degree of cavitation, thelower the whiteness and opacity. Yet another way to increase the opacityof the cavitated film is to add pigment opacifiers such as iron oxide,and as is more preferred, by using a lamellar structure pigment, such aslamellar graphite.

As indicated, some applications, such as VFFS packaging machines andsome HFFS packaging machines require higher density films that will passover the forming collars of the machine without damage due to filmcrazing or shearing of the package. It has been discovered that this canbe achieved by reducing the cavitation. To retain the film's pleasingaesthetic appearance, reducing the cavitation requires reducing thepigment volume as well, which results in loss of opacity and an increaseof light transmission. Loss of whiteness producing cavitation can beoffset by the use of TiO₂ in the intermediate and/or skin layers of thestructure each comprising 10% by weight of the total structure.

As indicated above, films which employ titanium dioxide-whitened outerskin layers do provide certain desirable benefits, particularly from anappearance standpoint. However, such films can also yield certainundesirable characteristics. These undesirable characteristics stem fromthe fact that titanium dioxide (TiO₂) is quite abrasive and, in fact,possess a hardness greater than even the chrome plating found on gravurerolls. This can result in excessive wear of expensive printing andcoating gravure roll surfaces, as well as any other surface which iscontacted by such a film. Other problems which arise from the use ofTiO₂ in the outer skin layers of such films is that fine deposits arelaid on converting machinery, extruder die lips, treater bar exhausts,etc. Also, appearance problems caused by streaks on the film, slippageon stretching either by roll or tentering can result.

The following specific examples are presented herein to illustrateparticular embodiments of the present invention and hence areillustrative of this invention and not to be construed in a limitingsense.

EXAMPLE 1

The film of this example was produced for comparison with the filmsproduced in the following examples.

A mixture of 92 percent, by weight, isotactic polypropylene (MP=320° F.,melt index=3), containing 8 weight percent PBT (MP=440° F.) as the corelayer void-initiating material, is melted in an extruder with a screw ofL/D ratio of 20/1 to provide the core layer mixture. A second extruder,in association with the first extruder, is supplied with the sameisotactic polypropylene as the first extruder, this extruder used toprovide the sink (intermediate) layer mixture. A melt coextrusion iscarried out while maintaining the cylinder of the core polymer materialat a temperature sufficient to melt the polymer mixture, i.e., fromabout 450° F. to about 550° F. or higher. The polypropylene mixtures ofthe second extruder to be used to form the skin (intermediate) layers ismaintained at about the same temperature as the polypropylene used infabricating the core layer. The mixture of the second extruder is splitinto two streams to enable the formation of skin layers on each surfaceof the core layer. As may be appreciated by those skilled in the art,rather than splitting the output of the second extruder into twostreams, a third extruder could be used to supply the second skin layermixture. Such an arrangement would be desired when the material used toform the second skin layer is varied from that of the first skin layer,when the thickness of the second skin layer is varied from that of thefirst skin layer, etc.

A three-layer film laminate was coextruded with a core thicknessrepresenting about 80 percent of the overall extruded thickness, withthe thicknesses of the skin layers representing about 20 percent of thefilm thickness. The resultant film sheet was subsequently oriented eightby five and three-quarter times using a commercially availablesequential biaxially orienting apparatus to provide a multi-layer filmstructure. The machine direction (MD) orientation is conducted at about285° F. and the transverse direction (TD) orientation is conducted atabout 300° F. The resultant multi-layer film exhibits a lustrous, whiteappearance and the following properties.

    ______________________________________                                        Optical gage (measured through microscope                                                                33 μm                                           Poly gage (Polypropylene equivalent)                                                                     22.4 μm                                         Density                    0.62 g/cc                                          Light transmission         22%                                                ______________________________________                                    

The degree of cavitation is indicated by film density, in g/cc, ascalculated by dividing the weight of one square meter of film by theoptical gage (μm).

EXAMPLE 2

This example demonstrates the effect on light transmission when lamellargraphite is added to the core mixture.

A mixture of 90 percent, by weight, isotactic polypropylene (MP=320° F.,melt index=3), containing 8 weight percent PBT (MP=440° F.), as the corelayer void-initiating material, and 2 percent lamellar graphite, ismelted in an extruder with a screw of L/D ratio of 20/1 to provide thecore layer mixture. A second extruder, in association with the firstextruder, is supplied with the same isotactic polypropylene as the firstextruder and about 1500 ppm of a finely divided silica antiblockingagent, this extruder used to provide the skin (intermediate) layermixture. A melt coextrusion is carried out while maintaining thecylinder of the core polymer material at a temperature sufficient tomelt the polymer mixture, i.e., from about 450° F. to about 550° F. orhigher. The polypropylene mixtures of the second extruder to be used toform the skin layers is maintained at about the same temperature as thepolypropylene used in fabricating the core layer. The mixture of thesecond extruder is split into two streams to enable the formation ofskin layers on each surface of the core layer.

A three-layer film laminate was coextruded with a core thickness againrepresenting about 80 percent of the overall extruded thickness, withthe thicknesses of the skin layers representing about 20 percent of thefilm thickness. The resultant film sheet was subsequently oriented eightby five and three-quarter times using a commercially availablesequential biaxially orienting apparatus to provide a multi-layer filmstructure. The machine direction (MD) orientation is conducted at about285° F. and the transverse direction (TD) orientation is conducted atabout 300° F. The resultant multi-layer film exhibits a pleasing,silvery appearance, not unlike aluminum foil, but containing no metaladditive. The properties of the film so produced were as follows.

    ______________________________________                                        Optical gage          35.5 μm                                              Poly gage             22.3 μm                                              Density               0.56 g/cc                                               Light transmission    1.25%                                                   ______________________________________                                    

Additionally the film of this example provides a very high barrier inthe visible to the ultra violet light range and is acceptable for use asa low density packaging material for processing on printing presses andpackaging machines.

EXAMPLE 3

This example demonstrates the effect of reduced cavitation, achievedthrough a reduction in machine direction orientation, on filmproperties.

As in Example 2 a mixture of 90 percent, by weight, isotacticpolypropylene (MP=320° F., melt index=3), containing 8 weight percentPBT (MP=440° F.), as the core layer void-initiating material, and 2percent lamellar graphite, is melted in an extruder with a screw of L/Dratio of 20/1 to provide the core layer mixture. A second extruder, inassociation with the first extruder, is supplied with the same isotacticpolypropylene as the first extruder and about 1500 ppm of a finelydivided silica antiblocking agent, this extruder used to provide theskin (intermediate) layer mixture. A melt coextrusion is carried outunder the same conditions as used in Example 2. Again, the mixture ofthe second extruder is split into two streams to enable the formation ofskin layers on each surface of the core layer.

A three-layer film laminate was coextruded with a core thickness againrepresenting about 80 percent of the overall extruded thickness, withthe thicknesses of the skin layers representing about 20 percent of thefilm thickness. The resultant film sheet was subsequently oriented eightby about five and one-third times using a commercially availablesequential biaxially orienting apparatus to provide a multi-layer filmstructure. The machine direction (MD) orientation is conducted at about290° F. and the transverse direction (TD) orientation is conducted atabout 300° F. The resultant multi-layer film exhibits an appearancewhich is darker, less attractive than the silvery appearance of the filmof Example 2. this is due to the reduction in whiteness and opacitywhich results from the reduction in cavitation. Other properties of thefilm are as follows.

    ______________________________________                                        Poly gage             22.3 μm                                              Optical gage          34 μm                                                Density               0.62 g/cc                                               Light transmission    2%                                                      ______________________________________                                    

EXAMPLE 4

This example demonstrates that the level of whiteness lost throughreduced cavitation, can be recovered through the use of a whiteningagent.

As in Examples 2 and 3, a mixture of 90 percent, by weight, isotacticpolypropylene (MP=320° F., melt index=3), containing 8 weight percentPBT (MP=440° F.), as the core layer void-initiating material, and 2percent lamellar graphite, is melted in an extruder with a screw of L/Dratio of 20/1 to provide the core layer mixture. A second extruder, inassociation with the first extruder, is supplied with the same isotacticpolypropylene as the first extruder, to which is added 4 percent TiO₂,this extruder used to provide the skin (intermediate) layer mixture. Amelt coextrusion is carried out under the same conditions as used inExample 2. Again, the mixture of the second extruder is split into twostreams to enable the formation of skin layers on each surface of thecore layer.

A three-layer film laminate was coextruded with a core thickness againrepresenting about 80 percent of the overall extruded thickness, withthe thicknesses of the skin layers representing about 20 percent of thefilm thickness. The resultant film sheet was subsequently oriented eightby about five and one-third times using a commercially availablesequential biaxially orienting apparatus to provide a multi-layer filmstructure. The machine direction (MD) orientation is conducted at about290° F. and the transverse direction (TD) orientation is conducted atabout 300° F. The resultant multi-layer film exhibits a pleasing,silvery appearance, essentially equivalent to that of the film ofExample 2, with a lighter appearance than the film of Example 2, and amuch lighter appearance than the film of Example 3. The properties ofthe film so produced are as follows:

    ______________________________________                                        Optical gage          33 μm                                                Poly gage             22.4 μm                                              Density               0.62 g/cc                                               Light transmission    2.2%                                                    ______________________________________                                    

EXAMPLE 5

The film of this example demonstrates the effect of adding encapsulatingskin layers to the film structure of Example 4 to form a five-layerstructure.

A mixture of 90 percent, by weight, isotactic polypropylene (MP=320° F.,melt index=3), containing 8 weight percent PBT (MP=440° F.), as the corelayer void-initiating material, and 2 percent lamellar graphite, ismelted in an extruder with a screw of L/D ratio of 20/1 to provide thecore layer mixture. A second and third extruder, in association with thefirst extruder, are each supplied with the same isotactic polypropylene(without PBT) as the first extruder, containing titanium dioxideparticles at 4 percent, by weight for this intermediate layer mixture. Afourth extruder, in association with the first three extruders, issupplied with the same isotactic polypropylene, without titaniumdioxide, to provide the skin layer mixture. A melt coextrusion iscarried out while maintaining the cylinder of the core polymer materialat a temperature sufficient to melt the polymer mixture, i.e., fromabout 450° F. to about 550° F. or higher. The polypropylene mixtures tobe extruded as intermediate layers are maintained at about the sametemperature as the polypropylene used in fabricating the core layer, asis the mixture being used for the skin layers. The mixture of the fourthextruder is split into two streams to enable the formation of skinlayers on each surface of the intermediate layers.

A five-layer film laminate was coextruded with a core thicknessrepresenting about 75 percent of the overall extruded thickness, withthe thicknesses of the intermediate layers representing about 20 percentand the skin layers representing about 5 percent of the film thickness.As in Example 4, the resultant film sheet was oriented eight by five andone-third times using a commercially available sequential biaxiallyorienting apparatus to provide a multi-layer film structure. The machinedirection (MD) orientation is conducted at about 290° F. and thetransverse direction (TD) orientation is conducted at about 300° F. Theresultant multi-layer film exhibits a smooth and lustrous appearance andexhibits the following properties.

    ______________________________________                                        Optical gage          33 μm                                                Poly gage             22.4 μm                                              Density               0.62 g/cc                                               Light transmission    2.2%                                                    ______________________________________                                    

EXAMPLE 6

This example demonstrates the effect of a yet further reduction incavitation on film properties, again achieved through a reduction inmachine direction orientation.

A five-layer film laminate, prepared in accordance with Example 5 wascoextruded with a core thickness representing about 75 percent of theoverall extruded thickness, with the thicknesses of the intermediatelayers representing about 20 percent and the skin layers representingabout 5 percent of the film thickness. For this Example, however, theresultant film sheet was oriented eight by four and three-quarter timesusing a commercially available sequential biaxially orienting apparatusto provide a multi-layer film structure. The machine direction (MD)orientation is conducted at about 295° F. and the transverse direction(TD) orientation is conducted at about 300° F. The resultant multi-layerfilm exhibits a smooth and lustrous appearance and exhibits thefollowing properties.

    ______________________________________                                        Optical gage          31 μm                                                Polygage              23.3 μm                                              Density               0.7 g/cc                                                Light transmission    3.8%                                                    ______________________________________                                    

EXAMPLE 6

The example demonstrates the effect of substituting a terpolymer forisotactic polypropylene to form the encapsulating skin layers of themulti-layer structure.

A mixture of 90 percent, by weight, isotactic polypropylene (MP=320° F.,melt index=3), containing 8 weight percent PBT (MP=440° F.), as the corelayer void-initiating material, and 2 percent lamellar graphite, ismelted in an extruder with a screw of L/D ratio of 20/1 to provide thecore layer mixture. The second and third extruders, in association withthe first extruder, were supplied with the same isotactic polypropylene(without PBT) as the first extruder, containing titanium dioxideparticles at 4 percent by weight for use in forming the intermediatelayer. A fourth extruder, in association with the first three extruders,was provided with an ethylene, 1-butene, polypropylene terpolymer,instead of isotactic polypropylene, together with 1500 ppm of a finelydivided silica antiblock agent. A melt coextrusion is carried out whilemaintaining the cylinder of the core polymer material at a temperaturesufficient to melt the polymer mixture, i.e., from about 450° F. toabout 550° F. or higher. Again, the polypropylene mixtures to beextruded as intermediate layers are maintained at about the sametemperature as the polypropylene used in fabricating the core layer, asis the terpolymer mixture being used to form the skin layers. As in theprevious Examples, the mixture of the fourth extruder is split into twostreams each to enable the formation of skin layers on each surface ofthe intermediate layer.

A five-layer film laminate is coextruded with a core thicknessrepresenting about 75 percent of the overall extruded thickness, withthe thicknesses of the intermediate layers representing about 20 percentand the skin layers representing about 5 percent of the film thickness.The resultant film sheet was subsequently oriented eight by four andthree-quarter times using a commercially available sequential biaxiallyorienting apparatus to provide a multi-layer film structure. The machinedirection (MD) orientation is conducted at about 295° F. and thetransverse direction (TD) orientation is conducted at about 300° F. theresultant multi-layer film exhibits the same appearance as the filmproduced in Example 5, with somewhat lower gloss. Properties determinedfor the film are as follows.

    ______________________________________                                        Optical gage           30 μm                                               Polygage               23 μm                                               Density                0.71 g/cc                                              Light transmission     4.0%                                                   Coefficient of friction                                                                              0.80                                                   ______________________________________                                    

As indicated, the film of this example has a terpolymer skin withantiblock for reducing surface friction. This film is most suitable forcoating after corona or flame treatment with a primer and subsequentlywith acrylic or polyvinylidene chloride to yield good machinability,sealability with good seal strength and hot tack properties.

EXAMPLE 7

This example demonstrates that the use of a migratory slip agent and amigratory antistatic agent in the intermediate layer blend reducessurface coefficient of friction characteristics.

A mixture of 90 percent, by weight, isotactic polypropylene (MP=320° F.,melt index=3), containing 8 weight percent PBT (MP=440° F.) as the corelayer void-initiating material and 2 percent lamellar graphite, ismelted in an extruder with a screw of L/D ratio of 20/1 to provide thecore layer mixture. (The lamellar graphite is again employed for itsbeneficial effect on reduced light transmission and overall filmappearance). A second and third extruder, in association with the firstextruder, are each supplied with the same isotactic polypropylene(without PBT) as the first extruder, but each containing titaniumdioxide particles at 4 percent, about 1200 ppm of Armostat® 410, anamine-based antistatic material, and about 1200 ppm of erucamide, byweight. A fourth extruder, in association with the first three extrudes,is supplied with the ethylene, 1-butene, polypropylene terpolymer ofExample 6, together with 1500 ppm of a finely divided silica antiblockagent and without titanium dioxide particles, Armostat® 410, anderucamide, this extruder being used to provide the skin layer mixture. Amelt coextrusion is carried out while maintaining the cylinder of thecore polymer material at a temperature sufficient to melt the polymermixture, i.e., from about 450° F. to about 550° F., or higher. Thepolypropylene mixtures of the second and third extruders to be extrudedas intermediate layers are maintained at about the same temperature asthe polypropylene used in fabricating the core layer, as are themixtures being used to for the skin layers. The mixture of the fourthextruder, again, is split into two streams to enable the formation ofskin layers on each surface of the intermediate layers.

A five-layer film laminate was coextruded with a core thicknessrepresenting about 75 percent of the overall extruded thickness, withthe thicknesses of the intermediate layers representing about 20 percentand the skin layers representing about 5 percent of the film thickness.The resultant film sheet was subsequently oriented eight by four andthree-quarter times using a commercially available sequential biaxiallyorienting apparatus to provide a multi-layer film structure. The machinedirection (MD) orientation is conducted at about 295° F. and thetransverse direction (TD) orientation is conducted at about 300° F. theresultant multi-layer film exhibits the same appearance as the film ofExample 6, with the following properties.

    ______________________________________                                        Optical gage           30 μm                                               Polygage               23 μm                                               Density                0.71 g/cc                                              Light transmission     4.0%                                                   Coefficient of friction                                                                              0.35*                                                  ______________________________________                                         *Measured after one week of aging to permit migration.                   

EXAMPLE 8

This example demonstrates that effect of an increase in film thicknesson the properties of a multi-layer film similar to the film of Example 7in other respects.

A five-layer film laminate, prepared in accordance with Example 7 wascoextruded to achieve an overall thicker film, again with a corethickness representing about 75 percent of the overall extrudedthickness, with the thicknesses of the intermediate layers representingabout 20 percent and the skin layers representing about 5 percent of thefilm thickness. The resultant multi-layer film exhibited the followingproperties.

    ______________________________________                                        Optical gage          54 μm                                                Polygage              38.5 μm                                              Density               0.7 g/cc                                                Light transmission    0.2%                                                    ______________________________________                                    

The coefficient of friction of this film is also about 0.35 after a weekof storage for migration.

As may be appreciated by those skilled in the art, the above examplesillustrate: the value of graphite added to a white cavitated core toproduce a high opacity film; the effect of the degree of cavitation onopacity and film appearance; that reduced cavitation improves filmsmachinability in HFFS and VFFS packaging machines and improves filmhandling and reduces the damage due to handling; that reduced cavitationmakes white opaque film less opaque and graphite pigmented opaque filmdarker and unpleasant in appearance; that the addition of TiO₂ willoffset the darkening effect to preserve the good appearance of the film;that adding copolymer outer skins in a five layer structure will improvethe sealability of the film when coated with acrylic on PvDC and the"soft" EP copolymer or terpolymer layer will reduce the core to skinlayer delamination and also improve hot tack; adding migratory slip andantistatic agents in the intermediate layers will produce migration tothe surface and reduce COF to a level on the order of 0.35.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be utilized without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the amended claims.

What is claimed is:
 1. An opaque, biaxially oriented polymer filmstructure, produced by orienting the film to a degree which provides lowlight transmission and improved machinability, comprising:(a) athermoplastic polymer matrix core layer having a first surface and asecond surface, within which is located a strata of voids; positioned atleast substantially within a substantial number of said voids is atleast one spherical void-initiating particle which is phase distinct andincompatible with said matrix material, the void space occupied by saidparticle being substantially less than the volume of said void, with onegenerally cross-sectional dimension of said particle at leastapproximating a corresponding cross-sectional dimension of said void;the population of said voids in said core being such as to cause asignificant degree of opacity, said core layer including a lightabsorbing pigment of lamellar morphology; (b) a first non-voidedthermoplastic polymer intermediate layer having a first surface and asecond surface, said second surface of said first intermediate layeradhering to said first surface of said core layer; and (c) a secondnon-voided thermoplastic polymer intermediate layer having a firstsurface and a second surface, said second surface of said secondintermediate layer adhering to said second surface of said core layer;herein the level of light transmission of the film is less than 10% andthe degree of orientation is about 8 TDO by less than about 5 MDO. 2.The film structure of claim 1, further comprising:(d) a first non-voidedthermoplastic skin layer adhering to said first surface of said firstintermediate layer; and (e) a second non-voided thermoplastic skin layeradhering to said first surface of said second intermediate layer.
 3. Thefilm structure of claim 2, wherein said void-free skin contains anantiblocking agent.
 4. The film structure of claim 1, wherein said corelayer is fabricated from isotactic polypropylene.
 5. The film structureof claim 4, wherein the void-initiating particles of said core layercomprise polybutylene terephthalate.
 6. The film structure of claim 1,wherein said intermediate layers are fabricated from isotacticpolypropylene.
 7. The film structure of claim 2, wherein said skinlayers are fabricated from isotactic polypropylene.
 8. The filmstructure of claim 1, wherein at least one of said intermediate layerscontain from about 2% to about 6% by weight of TiO₂.
 9. The filmstructure of claim 2, wherein said skin layers are fabricated from aheat sealable material.
 10. The film structure of claim 2, wherein saidskin layers are fabricated from a material selected from the groupconsisting of homopolymer of propylene, linear low density polyethylene,high density polyethylene, random copolymer of propylene and ethylene,block copolymer of propylene and ethylene, copolymer of propylene andbutylene, terpolymer of ethylene, propylene and butene, terpolymer ofethylene, propylene and butylene, and mixtures thereof.
 11. The filmstructure of claim 2 wherein said skin layers are fabricated from anethylene, 1-butene, propylene terpolymer.
 12. A process for preparing anopaque, biaxially oriented polymeric film structure of low lighttransmission and improved machinability, comprising the steps of:(a)mixing a major proportion of a first thermoplastic polymeric materialwith a minor proportion of a first material of higher melting point orhaving a higher glass transition temperature than the firstthermoplastic polymeric material to produce a core layer mixture and aminor amount of a light absorbing pigment of lamellar morphology; (b)heating the core layer mixture produced in step (a) to a temperature ofat least above the melting point of the first thermoplastic polymericmaterial; (c) dispersing the first material of higher melting point orhigher glass transition temperature of the mixture produced in step (a)uniformly throughout the molten first thermoplastic polymeric materialin the form of microspheres; (d) mixing a second thermoplastic polymericmaterial to produce a first intermediate layer mixture; (e) heating theintermediate layer mixture produced in step (d) to a temperature ofabout the melting point of the second thermoplastic polymeric material;(f) mixing a third thermoplastic polymeric material to produce a secondintermediate layer mixture; (g) heating the second intermediate layermixture produced in step (f) to a temperature of about the melting pointof the second thermoplastic polymeric material; and (h) forming abiaxially oriented coextruded film structure from the core layermixture, the first intermediate layer mixture and the secondintermediate layer mixture, said forming step conducted at a temperatureand to a degree to form a strata of opacifying voids within the corelayer; wherein the degree of orientation is about 8 TDO by less thanabout 5 MDO.
 13. The process of claim 12, wherein prior to conductingsaid forming step, at least one thermoplastic skin layer mixture isproduced for forming at least a first skin layer to encapsulate saidfirst intermediate layer.
 14. The process of claim 13, wherein a secondthermoplastic skin layer mixture is produced for forming a second skinlayer to encapsulate said second intermediate layer.
 15. The process ofclaim 14, wherein said skin layer mixtures contains an antiblockingagent.
 16. The process of claim 12, wherein said core layer mixtureincludes isotactic polypropylene.
 17. The process of claim 16, whereinthe void-initiating particles of said core layer comprise polybutyleneterephthalate.
 18. The process of claim 12, wherein said intermediatelayer mixtures include isotactic polypropylene.
 19. The process of claim14, wherein said skin layer mixtures include isotactic polypropylene.20. The process of claim 19, wherein said intermediate layer mixturesinclude from about 2 to about 6% by weight of TiO₂.